Water splitting system for hydrogen and oxygen separation in the absence of an ion exchange membrane

ABSTRACT

Systems and processes for the production of hydrogen (H2) gas and oxygen (O2) gas from an aqueous electrolyte solution are described. A water-splitting system can include a reactor that includes H2 and O2 generating chambers that can be separate chambers but are not separated by a H2 and/or O2 gas permeable material. The H2 generating chamber can include a cathode and at least a first fluid inlet. The O2 generating chamber can include an anode in electrical communication with the cathode and at least a first fluid inlet. The first and second fluid inlets can each be configured to receive a purged electrolyte solution, a purge gas, or a mixture thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/570,971 filed Oct. 11, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION A. Field of the Invention

The invention generally concerns generation of hydrogen (H₂) and oxygen (O₂) from an aqueous solution. This can performed by using a reactor that includes a H₂ generation chamber and an O₂ generation chamber. The reactor does not have to include a H₂ and O₂ permeable material such as an H₂ and O₂ permeable membrane or an ionic bridge.

B. Description of Related Art

Hydrogen (H₂) is a clean alternative to fuel. Conventional technology produces hydrogen on a commercial scale from steam reforming of methane. Due to the depletion of fossil fuels, there is a necessity to find an alternative feedstock to meet the growing demand for hydrogen production globally.

One alternative to methane steam reforming to produce hydrogen is through water-splitting. The reduction and oxidation half reactions for water-splitting are as follows:

2H⁺+2e ⁻

H₂  (1)

H₂O+2h ⁺

O₂+4H⁺  (2)

2H₂O

2H₂+O₂  (3)

Water-splitting can be achieved through electrolysis of water, photocatalytic splitting of water, or electrophotocatalytic splitting of water. These approaches are performed in acidic or basic media in conjunction with ion exchange membranes. The selection of the membrane can depend on the pH of the medium. For instance, a proton exchange membrane (PEM) can be used in an acidic environment, while an alkaline anion exchange membrane can used in a basic environment. Although membrane-based systems have high-energy efficiency and separate H₂/O₂ spontaneously, their application remains challenging by high cost and long-term stability. These cost and stability issues severely limit the commercial scalability of membrane-based systems. Hence, researchers have been investigating alternative methods. By way of example, Hashemi et al. (Energy Environ. Sci., 2015, 8, 2003) describes a membrane-less electrolyzer for hydrogen production across the pH scale. In this system, two parallel plates are coated with hydrogen and oxygen evolution catalysts, respectively, and are separated by less than a few hundreds of micrometers. The electrolyte flows between the catalyst plates and the evolved gases move close to the corresponding catalyst surface due to the Segre'-Silberberg effect. Each of the product gas streams can be collected in dedicated outlets. Stacks of these planes in horizontal can be used for higher throughput. Holmes-Gentle (Sustainable Energy Fuels, 2017, 1, 1184) describes a membrane-less photoelectrochemical cell similar to the Hashemi membrane-less electrolyzer. Both of these systems suffer in that the H₂/O₂ separation is only possible under supersaturating conditions right before the bubble formation. Further, the fact that there is only one reactor chamber where H₂ and O₂ are being produced can increase the possibility of creating an explosive H₂ and O₂ gas mixture exiting the reactor. In yet another example, U.S. Pat. No. 4,105,517 to Frosch describes a cyclic process for solar photolysis of water that includes production of H₂ from water in the presence of Eu⁺² photo-oxidizable reagent, pumping the resulting acidic solution to a second tank, where oxygen is generated in the dark during regeneration of the photocatalyst. This process suffers in that H₂ and O₂ are not produced simultaneously and requires regeneration of the process to be done in the absence of light. Further, the commercial scalability of these systems may not be economically feasible.

While various attempts to produce water-splitting systems have been made, they do not appear to meet the demands for commercial-scale production of H₂ and O₂ from water.

SUMMARY OF THE INVENTION

A discovery has been made that addresses at least some of the problems associated with currently available water-splitting processes. In one instance, the invention can address the problem associated with cross-contamination of H₂ and O₂ during the water-splitting process, and in particular, during the separation of H₂ and O₂ gases from the electrolyte solution. During this separation process, H₂ and O₂ combination can occur due to the continuous mixing of the aqueous solution with dissolved gasses. According to Henry's law, at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid; this is expressed by the following equation: C=k×P(gas), where C is the solubility of a gas at a fixed temperature in a particular solvent (in units of M or mL gas/L), k is Henry's law constant (often in units of M/atm), and P_(gas) is the partial pressure of the gas (often in units of Atm). In the context of the present invention, a reactor design has been discovered that can generate H₂ and O₂ under electrolysis and/or photoelectrical conditions in separate chambers and allows for the electrolyte solution to be purged and recycled to the reactor. In some embodiments, the H₂ and O₂ is generated simultaneously. This provides for an elegant flow through design with minimal cross-contamination of O₂ in the H₂ generating chamber and H₂ in the O₂ generating chamber. Notably, cross-contamination of H₂ in the O₂ generating chamber and O₂ in the H₂ generating chamber can be limited to less than 0.2 mol. %. Limiting cross-contamination can result in a H₂/O₂ oxygen mixture having H₂ and O₂ ratios under the explosion limit (5%). Still further, the reactor design can be operated in a cost efficient and safe manner that lends itself to commercial scale production of H₂ and/or O₂.

In a particular aspect of the invention a water-splitting system for the production hydrogen (H₂) gas and oxygen (O₂) gas from an aqueous electrolyte solution is described. The water-splitting system can include a reactor having H₂ and O₂ generating chambers that can be separate chambers but do not have to be separated by a H₂ and/or O₂ gas permeable material (e.g., a membrane, an ionic bridge, or both). The H₂ generating chamber can include a cathode and at least a first fluid inlet. The O₂ generating chamber can include an anode in electrical communication with the cathode and at least a first fluid inlet. The first fluid inlets of each of the H₂ and O₂ generating chambers can be fluidly coupled to a purged electrolyte source, a purge gas source, or a combination thereof. The first fluid inlets can each receive a purged electrolyte solution, a purge gas, or a combination thereof. The system can further include a H₂ reservoir fluidly coupled to the H₂ generating chamber. The H₂ reservoir can produce a H₂ containing gas stream and a H₂ containing electrolyte solution stream. In some embodiments, the system can further include a H₂ purification system that can be fluidly coupled with an H₂ outlet of the H₂ reservoir, preferably a H₂ permeable membrane. In some embodiments, the system can include an O₂ reservoir fluidly coupled to the O₂ generating chamber. The O₂ reservoir can produce an O₂ containing gas stream and an O₂ containing aqueous electrolyte solution stream. The system can further include an O₂ purification system that can be fluidly coupled to an O₂ outlet of the O₂ reservoir, preferably a O₂ permeable membrane. In some embodiments, an electrolyte source (e.g., a reservoir that contains the electrolyte source) can be fluidly coupled to each of the first fluid inlets. The electrolyte source can receive and purge the electrolyte solution from the H₂ generating chamber and/or the O₂ generating chamber. The purged electrolyte solution can be returned to the first fluid inlets of the H₂ generating and O₂ generating chambers using a fluid mover (e.g., pumped or pressurized to the chambers). In some embodiments, the first fluid inlets can be purge gas inlets. In another embodiment, the first fluid inlets can receive the purged electrolyte solution, and the H₂ generating chamber and the O₂ generating chamber each can further include a second fluid inlet. Each second fluid inlet can be fluidly coupled to a purge gas source. In a preferred embodiment, the reactor can be a flow-through reactor. In a preferred embodiment, the anode and the cathode are included in a H₂ or O₂ impermeable material positioned at least partially between or substantially between the H₂ generating chamber and the O₂ generating chamber. The anode can include an oxidation catalyst, preferably a H₂ generating photocatalyst in fluid communication with the aqueous electrolyte solution in the H₂ generating chamber. The H₂ generating chamber can receive electromagnetic radiation, which can be used to excite the photocatalyst, which in turn catalyzes the production of H₂ and holes. The cathode can include a reduction catalyst, preferably an O₂ generating photocatalyst, in fluid communication with the aqueous electrolyte solution in the O₂ generating chamber. The O₂ generating chamber can receive electromagnetic radiation, which can be used to excite the photocatalyst, which in turn catalyzes the production of O₂ and electrons. The produced electrons can be transferred to the anode via an electrical connection between the cathode and anode (e.g., conductive material such as a conductive wire). In some embodiments, the H₂ generating chamber can be fluidly coupled to the O₂ generating chamber. The H₂ generating chamber can be coupled to the O₂ generating chamber by one or more apertures. In some embodiments, first and second apertures are comprised in a conduit that connects the H₂ generating chamber with the O₂ generating chamber. The apertures or conduit can be positioned in the lower portion of the two chambers to allow transport of ions into each chamber with limited or without cross-contamination of H₂ and O₂ into the O₂ and H₂ generating chambers, respectively. The lower portion of the H₂ and O₂ generating chambers can include apertures that are on or in the side walls of each chamber and that are positioned anywhere in the bottom half of the reactor such as at half the height of the chambers or less. In some instances, the apertures or conduit can be positioned on the side wall of each chamber proximate the bottom of the chamber. In some embodiments, the aperture is a hole or a plurality of holes (e.g., a screen) in the H₂ and O₂ impermeable material that separates the H₂ generating chamber with the O₂ generating chamber.

In another aspect of the invention a water-splitting process for production of H₂ and O₂ is described. The process can include providing an electrolyte solution to any of the water-splitting systems of the present invention. The electrolyte solution can include, water, a purge gas, and an electrolyte. The electrolyte solution in the H₂ generating chamber and the electrolyte solution in the O₂ generating chamber can be subjected to conditions sufficient to produce a H₂ containing aqueous electrolyte solution and an O₂ containing aqueous electrolyte solution. The purge gas, at least a portion of the generated H₂, and at least a portion of the generated O₂ can be dissolved in each of the aqueous electrolyte solutions. In certain instances, a majority of or all of the generated H₂ and/or generated O₂ can be dissolved in the aqueous electrolyte solution. The H₂ containing aqueous electrolyte solution and/or the O₂ containing aqueous electrolyte solution can be subjected to conditions suitable to produce a purge gas containing aqueous electrolyte solution, a gaseous H₂ stream, and a gaseous O₂ stream. In some embodiments, the purge gas containing aqueous electrolyte solution can be provided to the H₂ generating chamber of the water-splitting system, the O₂ generating chamber of the water-splitting system, or both. The purge gas containing aqueous electrolyte solution can be free of H₂ and O₂. In other instances, the purge gas containing aqueous electrolyte solution can include H₂ and O₂ in molar H₂/O₂ ratio under the flammability limit. The purge gas can by any gas, preferably an inert gas, most preferably nitrogen N₂. The purge gas can reduce or limit contamination of H₂ into the O₂ containing aqueous electrolyte stream, O₂ into the H₂ containing aqueous electrolyte stream, or both. Water-splitting conditions can include a pressure of 0.010 MPa to 2.1 MPa, a temperature of 5° C. to 100° C., a pH of 0 to 14, or a combination thereof. Step (c) can include compressing the H₂ containing aqueous electrolyte solution stream to produce a gaseous H₂ stream and the electrolyte solution and/or (i) collecting the H₂ containing aqueous electrolyte solution stream in the H₂ reservoir, the O₂ containing aqueous electrolyte solution stream in the O₂ reservoir or both; (ii) separating the H₂ gaseous stream from the H₂ containing aqueous electrolyte solution and the O₂ gaseous stream from the O₂ containing aqueous electrolyte solution, or both; (iii) forming an aqueous electrolyte solution comprising residual H₂, O₂, or both; and (iv) purging the step (iii) aqueous electrolyte solution with the purge gas to form the purge gas containing aqueous electrolyte solution. In some embodiments, the process includes providing a purge gas to the H₂ generating chamber, the O₂ generating chamber, or both and/or providing the purged electrolyte solution to the H₂ generating chamber and the O₂ generating chamber.

In the context of the present invention, 20 embodiments are described. Embodiment 1 describes a water-splitting system for the production of hydrogen (H₂) gas and oxygen (O₂) gas from an aqueous electrolyte solution, the system comprising a reactor comprising: a H₂ generating chamber comprising a cathode and at least a first fluid inlet fluidly coupled to a purged electrolyte source, a purge gas source, or a combination thereof, and an O₂ generating chamber comprising an anode in electrical communication with the cathode and at least a first fluid inlet fluidly coupled to the purged electrolyte source, the purge gas source, or a combination thereof, wherein the H₂ and O₂ generating chambers are not separated by a H₂ or an O₂ gas permeable material. Embodiment 2 is the water-splitting system of embodiment 1, wherein the H₂ or O₂ gas permeable material is a membrane, an ionic bridge, or both. Embodiment 3 is the water-splitting system of any one of embodiments 1 to 2, further comprising a H₂ reservoir fluidly coupled to the H₂ generating chamber, the purged electrolyte source and an H₂ product outlet. Embodiment 4 is the water-splitting system of embodiment 3, further comprising a H₂ purification system, preferably a H₂ permeable membrane, fluidly coupled to the H₂ product outlet. Embodiment 5 is the water-splitting system any one of embodiments 1 to 4, further comprising an O₂ reservoir fluidly coupled to the O₂ generating chamber, the purged electrolyte source, and an O₂ product outlet. Embodiment 6 is the water-splitting system of embodiment 5, further comprising an O₂ purification system, preferably an O₂ permeable membrane, fluidly coupled to the O₂ product outlet. Embodiment 7 is the water-splitting system of any one of embodiments 1 to 6, wherein purged electrolyte source is fluidly coupled to the purge gas source. Embodiment 8 is the water-splitting system of any one of embodiments 1 to 7, wherein the first fluid inlets are fluidly coupled to the purged electrolyte source, and wherein the H₂ generating chamber further comprises a second inlet and/or the O₂ generating chamber further comprises a second inlet, each second inlet fluidly coupled to the purge gas source. Embodiment 9 is the water-splitting system of any one of embodiments 1 to 8, wherein the H₂ generating chamber and the O₂ generating chamber are fluidly coupled by at least one aperture positioned at a lower portion of both chambers, the at least one aperture sized to allow transport of ions between each chamber. Embodiment 10 is the water-splitting system of any one of embodiments 1 to 9, further comprising a conduit coupled to the H₂ generating chamber and the O₂ generating chamber, the conduit comprising a first aperture coupled to the H₂ generating chamber and a second aperture coupled to the O₂ generating chamber. Embodiment 11 is the water-splitting system of any one of embodiments 1 to 10, wherein the anode and the cathode are comprised in a H₂ and/or O₂ gas impermeable material positioned at least partially between the H₂ generating chamber and the O₂ generating chamber. Embodiment 12 is the water-splitting system of embodiment 11, wherein the anode comprises a photo-reduction catalyst in fluid communication with a purged electrolyte solution from the purged electrolyte source, and the H₂ generating chamber is in communication with an electromagnetic radiation source. Embodiment 13 is the water-splitting system of any one of embodiments 11 to 12, wherein the cathode comprises a photo-oxidation catalyst in fluid communication with a purged electrolyte solution from the purged electrolyte source, and the O₂ generating chamber is in communication with an electromagnetic radiation source. Embodiment 14 is the water-splitting system of any one of embodiments 11 to 13, wherein the H₂ generating chamber and the O₂ generating chamber are fluidly coupled by at least one aperture positioned at a lower portion of both chambers, and wherein the at least one aperture is comprised in the H₂ and/or O₂ gas impermeable material.

Embodiment 15 is a water-splitting system for the production of hydrogen (H₂) gas and oxygen (O₂) gas from an aqueous electrolyte solution, the system comprising: a reactor comprising: a H₂ generating chamber comprising a cathode and at least a first fluid inlet fluidly coupled to a purged electrolyte source, a purge gas source, or a combination thereof; and an O₂ generating chamber fluidly coupled to the H₂ generating chamber, the O₂ generating chamber comprising an anode in electrical communication with the cathode and at least a first fluid inlet fluidly coupled to the purged electrolyte source, the purge gas source, or a combination thereof, wherein the H₂ and O₂ generating chambers are not separated by a H₂ or an O₂ gas permeable material.

Embodiment 16 is a water-splitting process for the production of hydrogen (H₂) gas and oxygen (O₂) gas, the process comprising: (a) providing an electrolyte solution to each of the H₂ generating chamber and the O₂ generating chamber of the water-splitting system of any one of embodiments 1 to 15, the electrolyte solution comprising water, a purge gas, and an electrolyte; (b) subjecting the electrolyte solution in the H₂ generating chamber and the electrolyte solution in the O₂ generating chamber to conditions sufficient to produce a H₂ containing electrolyte solution in the H₂ generating chamber and an O₂ containing electrolyte solution in the O₂ generating chamber, wherein at least a portion of the generated H₂ is dissolved in the H₂ containing aqueous electrolyte solution, and at least a portion of the generated O₂ is dissolved in the O₂ containing electrolyte solution; and (c) subjecting the H₂ containing electrolyte solution and/or the O₂ containing electrolyte solution to conditions suitable to produce a purge gas containing electrolyte solution, a gaseous H₂ stream, a gaseous O₂ stream, or combinations thereof. Embodiment 17 is the process of embodiment 16, further comprising providing the purge gas containing electrolyte solution to the H₂ generating chamber of the water-splitting system, the O₂ generating chamber of the water-splitting system, or both, wherein the purge gas containing electrolyte solution comprises H₂ and O₂ in a molar H₂/O₂ ratio under the explosion limit. Embodiment 18 is the process of any one of embodiments 16 to 17, wherein the purge gas comprises any gas, preferably an inert gas, most preferably nitrogen N₂, and wherein the purge gas is at least partially or fully solubilized in the aqueous electrolyte solution. Embodiment 19 is the process of any one of embodiments 16 to 18, wherein the water-splitting conditions comprise a pressure of 0.010 MPa to 2.1 MPa, a temperature of 5° C. to 100° C., a pH of 0 to 14, or a combination thereof. Embodiment 20 is the process of any one of embodiments 16 to 19, wherein the purge gas reduces contamination of H₂ into the O₂ containing aqueous electrolyte stream, O₂ into the H₂ containing aqueous electrolyte stream, or both. Embodiment 21 is the process of any one of embodiments 16 to 20, wherein step (c) comprises compressing the H₂ containing aqueous electrolyte solution stream to produce a gaseous H₂ stream and the electrolyte solution comprising the purge gas. Embodiment 22 is the process of any one of embodiments 16 to 21, wherein step (c) comprises: (i) collecting the H₂ containing aqueous electrolyte solution stream in the H₂ reservoir, the O₂ containing aqueous electrolyte solution stream in the O₂ reservoir or both; (ii) separating the H₂ gaseous stream from the H₂ containing aqueous electrolyte solution and the O₂ gaseous stream from the O₂ containing aqueous electrolyte solution, or both; (iii) forming an aqueous electrolyte solution comprising residual H₂, O₂, or both; and (iv) purging the step (iii) aqueous electrolyte solution with the purge gas to form the purge gas containing aqueous electrolyte solution. Embodiment 23 is the process of any one of embodiments 16 to 22, further comprising: providing a purge gas to the H₂ generating chamber, the O₂ generating chamber, or both; and providing the purged electrolyte solution to the H₂ generating chamber and the O₂ generating chamber. Embodiment 24 is the process of any one of embodiments 16 to 23, wherein step (b) further comprises flowing the a portion of the electrolyte solution between the H₂ generating chamber and the O₂ generating chamber through at least one of the apertures of embodiments 10, 11, or 15.

The following includes definitions of various terms and phrases used throughout this specification.

The phrase “electromagnetic radiation” refers to all wavelengths of light unless specified otherwise. Non-limiting example of wavelengths of light include radio wave, microwave, infrared, visible light, ultraviolet, X-ray, and gamma radiation, or any combination thereof. In some preferred instances, the electromagnetic radiation can include visible light or ultraviolet light or a combination of the two.

The term “coupled” is defined as connected, although not necessarily directly, and not necessarily mechanically; two items that are “coupled” may be unitary with each other.

The terms “about” or “approximately” are defined as being close to the value, term, or phrase that follows, as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

The terms “wt. %,” “vol. %,” or “mol. %” refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt. % of component.

The term “substantially” and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

The terms “inhibiting” or “reducing” or “preventing” or “avoiding” or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

The term “effective,” as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

The use of the words “a” or “an” when used in conjunction with any of the terms “comprising,” “including,” “containing,” or “having” in the claims, or the specification, may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”

The words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

The photoelectrochemical systems of the present invention can “comprise,” “consist essentially of,” or “consist of” particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase “consisting essentially of,” in one non-limiting aspect, a basic and novel characteristic of the photoelectrochemical water-splitting systems of the present invention are their abilities to produce high purity H₂ and O₂ in a continuous manner. In certain instances, the H₂ and O₂ generating chambers can be separate chambers, but are not separated by H₂ and O₂ permeable membranes and/or an ionic bridge.

Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

FIG. 1 depicts a schematic of the water-splitting system of the present invention.

FIG. 2 depicts a schematic of the water-splitting system of the present invention with a H₂ reservoir and an O₂ reservoir.

FIG. 3A depicts a schematic of the water-splitting system of the present invention of FIG. 1 with an aperture between the H₂ generating chamber and the O₂ generating chamber.

FIG. 3B depicts a schematic of the water-splitting system of the present invention of FIG. 2 with an aperture between the H₂ generating chamber and the O₂ generating chamber.

FIG. 4A depicts a schematic of the water-splitting system of the present invention with a H₂ generating chamber and the O₂ generating chamber and a H₂ and O₂ gas impermeable material positioned between the chambers.

FIG. 4B depicts a schematic of the water-splitting system of FIG. 4A with an aperture in the H₂ and O₂ gas impermeable material.

FIGS. 5A through 5D depict a (5A) front view of a stacked water-splitting reactor, (5B) back view of the stacked water-splitting reactor, (5C) gas impermeable material with anodic material, and (5D) gas impermeable material with cathodic material.

FIGS. 6A through 6D depict a (6A) front view of a stacked water-splitting reactor having a purge gas inlet, an electrolyte source inlet and a gas outlet, (6B) back view of the stacked water-splitting reactor, (6C) gas impermeable material with anodic material, (6D) gas impermeable material with cathodic material.

FIG. 7 depicts a schematic of a monolithically integrated photocatalyst catalyst configuration of the water-splitting system of the present invention.

FIGS. 8A and 8B depict schematics of photocatalyst catalyst configurations of the water-splitting system of the present invention.

FIGS. 9A and 9B are graphs of addition of H₂ (FIG. 9A) and O₂ (FIG. 9B) into a H₂ reservoir and O₂ reservoir of the system of the present invention.

FIGS. 10A and 10B are graphs of moles of H₂ and O₂ after injecting both separately to a H₂ reservoir and O₂ reservoir of the system of the present invention as a function of time.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

A discovery has been made that provides a solution to the inefficiencies of water-splitting systems (e.g., PEC systems and/or an electrolysis systems). The discovery is premised on a reactor that does not require the use of a H₂ and/or O₂ gas permeable material such as a membrane or an ionic bridge. In lieu of such a membrane, a reactor of the present invention can provide purged electrolyte solution to a hydrogen generating chamber and an oxygen generating chamber with a minimal amount of H₂ or O₂ cross contamination in the respective chambers.

These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the FIGS. The systems and methods of described in FIGS. 1 to 6 can also include various equipment that is not shown and is known to one of skill in the art of chemical processing. For example, some controllers, piping, computers, valves, pumps, heaters, thermocouples, pressure indicators may not be shown.

A. Water-Splitting Systems

Referring to FIG. 1, a schematic of water-splitting system 100 of the present invention source. Water-splitting reactor 102 can include H₂ generating chamber 106 and O₂ generating chamber 108. In some embodiments, the chambers can receive electromagnetic radiation. In preferred instances, the electromagnetic radiation can be visible light (e.g., sunlight or artificially produced visible light) or ultraviolet radiation or a combination of the two. By way of example, reactor 102, H₂ generating chamber 106, and O₂ generation chamber 108 can include one or portions or sides that are transparent to light, either sunlight, or artificial light. Non-limiting examples of transparent materials include glass, quartz, organic polymers, silica-based polymers and the like. Alternatively, chamber 106 or 108 can be opaque and a light source can be placed within said chambers, affixed to the walls of said chambers, and/or built into the walls of said chambers (not shown). Reactor 102 can include a housing 110 (shown with dotted lines) that houses H₂ generation chamber 106 and O₂ generation chamber 108. Housing and chambers can include spacers and connectors suitable to position the chambers in the housing (See, for example FIGS. 4A-4D). In some embodiments, a portion or all of housing 110 can be transparent. In some embodiments, the chambers are aligned in a parallel configuration in the housing such that electromagnetic radiation passes through the housing, a chamber, and into the adjacent chamber. In some embodiments, a housing is not necessary and/or sides of the housing make up the sides of the chambers. As shown, H₂ generating chamber 106 and O₂ generating chamber 108 have four distinct sides. However, H₂ generating chamber 106 and O₂ generating chamber 108 can have a shared wall that is impermeable to H₂ and O₂ gases. In some embodiments, all or portions of the reactor 102, housing 110, H₂ generating chamber 106, and O₂ generation chamber 108 can be manufactured from a polymeric material (e.g., polymethylmethacrylate (PMMA)). H₂ generating chamber 106 can include cathode 112 capable of reducing H⁺ ions in electrolyte solution 114. In some embodiments, cathode 112 is a photocatalyst capable of accepting electromagnetic radiation and catalyzing the generation of H₂ from water and electrolyte solution 114. H₂ generating chamber 106 can include H₂ outlet 116 and electrolyte solution outlet 118. H₂ outlet 116 can allow generated H₂ to be removed from the H₂ generating chamber and be in fluid communication with purification systems and/or collection systems (not shown). Electrolyte solution outlet 118 can be in fluid communication with electrolyte source 104 via piping 120 to allow H₂ containing electrolyte solution to be removed from H₂ generating chamber 106 and be provided to electrolyte source 104.

O₂ generating chamber 108 can include anode 124 capable of oxidizing OH⁻ in electrolyte solution 114 to O₂. In some embodiment anode 124 include a photocatalyst capable of catalyzing generation of O₂ from water and electrolyte solution 114. Anode 124 and cathode 112 can be electrically coupled through circuit 126. Circuit 126 can be a wire (e.g., copper wire) that connects the two electrodes. In some embodiments, circuit 126 can include a power source to supply electricity to one or more electrons. It should be understood, that one of skill in the art can electrically connect the cathode and anode as needed depending on the chosen electrode or catalyst. O₂ generating chamber 108 can include O₂ outlet 128 and electrolyte solution outlet 130. O₂ outlet 128 can allow generated O₂ to be removed from the O₂ generating chamber and be in fluid communication with purification systems and/or collection systems (not shown). Electrolyte solution outlet 130 can be in fluid communication with electrolyte source 104 via piping 132 to allow O₂ containing electrolyte solution to be removed from O₂ generating chamber 108 and be provided to electrolyte source 104.

Electrolyte source 104 can include electrolyte solution inlet 134, purge gas (e.g., N₂, argon, inert gas, or other gases) inlet 136, and electrolyte solution outlet 138. Electrolyte solution inlet 134 can be in fluid communication with piping 120 and/or other piping that allow gas containing (e.g., H₂ and/or O₂) electrolyte solutions and/or fresh electrolyte solution to enter electrolyte source 104. Purge gas inlet 136 can be in fluid communication with sparging system (not shown) capable of delivering a sufficient amount of purge gas to substantially or completely remove (degas) dissolved reactive gases (e.g., H₂ and/or O₂), forming degassed electrolyte solution 140. In certain instances, the purge gas can be any gas that does not react with the water-splitting materials or reagents (e.g., cathode material, anode material, intermediate reactants, products, or water). Non-limiting examples of purge gas include nitrogen (N₂), helium (H₂), argon (Ar), carbon dioxide (CO₂), hydrocarbon gases (e.g., methane, ethane, propane and butane). In a preferred embodiment, N₂ is used as the purge gas. System 100 can include fluid mover 146 (e.g., a pump). Degassed electrolyte solution 140 can be moved using fluid mover 146 to H₂ generating chamber electrolyte inlet 142, O₂ generating chamber inlet 144, via degassed electrolyte solution outlet 138 and piping 148, 150, and 152. Piping 148, 150, and 152 can fluidly couple the H₂ generating chamber with the O₂ generating chamber. In some embodiments, pressure from purge gas entering inlet 136 is sufficient to move the electrolyte solution to the various chambers. Removing the H₂ and O₂ from the electrolyte solution can minimize or inhibit cross-contamination of H₂ into the O₂ generating chamber and/or O₂ into the H₂ generating chamber. Such cross contamination can cause formation of water molecules from reactions of H⁺ and or OH⁻ with the generated O₂, H₂ respectively. The reactive gas mixture can be removed from the electrolyte source via reactive gas outlet 154. Reactive gas mixture can be a mixture of H₂, O₂ and purge gas and have a molar H₂ to O₂ ratio under the flammability limit. Reactive gas outlet 154 can be in fluid communication with a collection unit, purification unit, transportation line, or the like. In some embodiments, system 100 is an electrolysis system or a photoelectrochemical system.

In some embodiments, the water-splitting system includes a H₂ reservoir and an O₂ reservoir fluidly coupled to the electrolyte source and the H₂ generating chamber and the O₂ generating chamber. Inclusion of H₂ and O₂ reservoirs can allow for separation of the H₂ gas and/or O₂ gas from the H₂ and O₂ containing electrolyte solutions prior to the electrolyte solution entering electrolyte source 104. Referring to FIG. 2A a schematic of water-splitting system 200 of the present invention is depicted that includes H₂ collection unit (reservoir) 202 and O₂ collection unit (reservoir) 204. Although FIG. 2A depicts both reservoirs, it should be understood that one reservoir could be used (e.g., a H₂ reservoir and no O₂ reservoir or vice versa). System 200 shown in FIG. 2 includes water-splitting reactor 102, electrolyte source 104 and other components of FIG. 1. H₂ reservoir 202 can be in fluid communication with H₂ generating chamber 106 of reactor 102 via H₂ outlet 116, piping 206, and reservoir H₂ inlet 208. As H₂ is generated in H₂ generation chamber 106, a mixture of electrolyte solution with dissolved and free H₂ can exit the H₂ generation chamber and enter H₂ reservoir. In H₂ reservoir 202, aqueous electrolyte droplets can separate from the gaseous H₂. In some embodiments, the purge gas can enter H₂ generating chamber through inlet 142 or through a second inlet (not shown) and purge or sweep gaseous H₂ from H₂ generating chamber 106 into the H₂ reservoir 202. Addition of the purge gas into H₂ generating chamber 106 can saturate the cathode with purge gas and inhibit H⁺ ions present to combine with any OH⁻ present to form H₂O. Gaseous H₂ can exit H₂ reservoir 202 via H₂ outlet 210 and be in fluid communication with H₂ permeable membrane 212 via piping 214. Membrane 212 can be a H₂ permeable membrane capable of separating H₂ from the purge gas and trace amounts of O₂.

O₂ reservoir 204 can be in fluid communication with O₂ generating chamber 108 of reactor 102 via O₂ outlet 128, piping 218, and reservoir O₂ inlet 220. As O₂ is generated in O₂ generation chamber 108, a mixture of electrolyte solution with dissolved and free O₂ can exit the O₂ generation chamber and enter O₂ reservoir. In O₂ reservoir 204, aqueous electrolyte droplets separate from the gaseous O₂. In some embodiments, the purge gas can enter O₂ generating chamber 108 through inlet 144 or a second inlet (not shown) and purge or sweep gaseous O₂ from O₂ generating chamber 108 into O₂ reservoir 204. Addition of the purge gas into O₂ generating chamber 106 can saturate the anode with purge gas and inhibit OH-ions present to combine with any H⁺ present to form H₂O. Gaseous O₂ can exit O₂ reservoir 204 via O₂ outlet 222 and be in fluid communication with O₂ permeable membrane 224 via piping 226. Membrane 224 can be an O₂ permeable membrane capable of separating O₂ from the purge gas and/or trace amounts of H₂.

H₂ reservoir 202 and O₂ reservoir 204 can be in fluid communication with electrolyte source inlet 146 via piping 228 and 230, respectively. Electrolyte source 104 can receive electrolyte solution from H₂ reservoir 202, O₂ reservoir 204, or both. Such an electrolyte solution can have dissolved H₂ and O₂ in the solution. The dissolved H₂ and O₂ can be removed from the electrolyte solution to produce a degassed electrolyte solution that can be returned to H₂ generating chamber and O₂ generating chamber via piping 146, 148 and 150 as described for system 100.

In some embodiments, the H₂ generating chamber and the O₂ generating chamber can be in direct fluid communication with each other. By way of example, the two chambers can include an aperture that connects the two chambers. The aperture can be any size or shape (e.g., parabolic, circular, elliptical, trapezoid, parallelogram, square, rectangular, polygonal, or the like). The aperture can be sized to be sufficient to allow mass transport of ions (H⁺) and (OH⁻) at a rate sufficient to sustain a water-splitting reaction. Such sizing can be determined by known engineering methods depending on the size of the reactor. FIG. 3A depicts system 300 that includes the water-splitting system of FIG. 1 having the H₂ generating chamber directly coupled with O₂ generating chamber via an aperture. FIG. 3B system 300 that includes the water-splitting system of FIG. 2 having the H₂ generating chamber directly coupled with O₂ generating chamber via an aperture. In FIGS. 3A and 3B, a first aperture 302 and a second aperture 304 are included in conduit 306. In some embodiments, first aperture 302 and second aperture 304 can be a single hole or a plurality of holes (e.g., a screen). Fluid can equilibrate between the two chambers so that the solutions remain pH neutral. As shown the aperture(s) are positioned at the lower portion of the chambers. Such a positioning can allow for minimal amount of cross-contamination of H₂ and O₂ in the respective chambers as H₂ and O₂ have low solubility in water and upward in the solution to H₂ outlet/electrolyte solution 116 and O₂ outlet/electrolyte solution 130. The exact positioning of aperture(s) within the lower portion of the chambers can be modified as desired and they can be placed in the lower half of the side walls of the chambers relative to the height of each chamber. In preferred instances, the apertures 302 and 303 and conduit 306 can be placed in the side walls of the chamber proximate to the bottom of the chambers. In some embodiments, apertures 302 and 304 can include covers (not shown) that can cover the apertures during use. By way of example, the apertures can be covered when cross-contamination of O₂ is detected in the H₂ generating chamber or cross-contamination of H₂ is detected in the O₂ generating chamber. In some embodiments, the covers can be electronically controlled. In system 100 of FIGS. 3A and 3B, circuit 126 can be connected through conduit 302 (not shown) instead of outside the reactor.

In some embodiments of the present invention, the water-splitting system includes a H₂ and/or O₂ impermeable material. Referring to FIGS. 4A and 4B, a schematic of water-splitting reactor 102 having H₂ generating chamber 106, O₂ generating chamber 108, and H₂ and/or O₂ impermeable material 402 positioned between the two chambers. Referring to FIG. 4B, impermeable material 402 includes aperture 404 that allows H₂ generating chamber and O₂ generating chamber to be fluidly coupled. As discussed for FIG. 3B, aperture can be a hole or a plurality of holes that allows transport of ions into each chamber. Reactor 102 of system 400 can be coupled to electrolyte source 104, H₂ reservoir 202, O₂ reservoir 204 as described in FIGS. 1, 2, 3A and 3B. FIGS. 4A and 4B H₂ and/or O₂ impermeable material 302 can be made of the same materials as H₂ generating chamber 106, O₂ generating chamber 108, and/or housing 110. In some embodiments, H₂ or O₂ impermeable material is an inner wall of both chambers. A portion of the surface of H₂ or O₂ impermeable material 302 in the H₂ generating chamber can include the cathode and/or a photocatalyst capable of generating H₂ from water. In some embodiments, the cathode material and/or the photocatalyst can be deposited on the surface or coated on the surface of the impermeable material. The opposite surface of H₂ or O₂ impermeable material 302 in the O₂ generating chamber can be include the anode and/or photocatalyst capable of generating O₂ from water. In some embodiments, the anode and/or photocatalyst can be deposited on the surface or coated on the surface of the impermeable material. In some embodiments, circuit 126 is not necessary. H₂ and O₂ generated in their respective chambers can be captured as previously described. The electrolyte solution from the chambers can be in fluid communication with the electrolyte source as previously described.

In some embodiments, the water-splitting system of the present invention can have a stacked configuration as shown in FIGS. 5A-5D. FIG. 5A is a front view of the stacked water-splitting reactor of the present invention that includes H₂ generating chamber 106, electrolyte inlet 142, and H₂/electrolyte solution outlet 116. FIG. 5B is a back view of a stacked water-splitting reactor that includes O₂ generating chamber 108, electrolyte inlet 138, and O₂/electrolyte solution outlet 128. Gas impermeable material 302 can be positioned between the two chambers and can include a cathode material and/or photocatalyst (e.g., 112) and an anode material and/or photocatalyst (e.g., 124) as shown in FIGS. 5C and 5D. Cathode material 112 can be in fluid communication with electrolyte solution flowing into inlet 142. Cathode material 124 can be in fluid communication with electrolyte solution flowing into inlet 138. Spacers 502 and 504 can be positioned between the gas impermeable material and the H₂ generating chamber and the O₂ generating chamber, respectively. As shown, reactor 102 includes transparent region 506 allows light to be transmitted through the cell to the photocatalysts. The reactor can be any suitable size for performing a water-splitting reaction. In one embodiment, the reactor can be 10 cm×4.8 cm×4.9 cm. The volume ratio of the H₂ generating chamber 106 and the volume of O₂ generating chamber 108 to the H₂ and O₂ reservoir 202 and 204, and the electrolyte reservoirs 104 can be 1:90:20 to 1:100:30, or 1:95:25, or any ratio there between.

In some embodiments, the water-splitting system of the present invention can have a reactor having a stacked configuration that includes inlets for purge gas and electrolyte solution and outlets for H₂ containing electrolyte solution and O₂ containing electrolyte solution as shown in FIGS. 5A-5D. FIG. 5A is a front view of the stacked water-splitting reactor of the present invention that includes H₂ generating chamber 106, electrolyte inlet 142, H₂/electrolyte solution outlet 116, and purge gas inlet 502. FIG. 5B is a back view of a stacked water-splitting reactor O₂ generating chamber 108, electrolyte inlet 138, and O₂/electrolyte solution outlet 128. In some embodiments, H₂ generating chamber 106 include electrolyte inlet 142 and H₂/electrolyte solution outlet 116, and purge gas inlet 502 and O₂ generating chamber 108 includes electrolyte inlet 138, O₂/electrolyte solution outlet 128, and purge gas inlet 502. In another embodiment, both chambers include the purge gas inlet 602. Gas impermeable material 302 can be positioned between the two chambers and includes a cathode material and/or photocatalyst (e.g., catalyst 112) and an anode material and/or photocatalyst (e.g., catalyst 124) as shown in FIGS. 6C and 6D. Cathode material 112 can be in fluid communication with electrolyte solution flowing into inlet 142. Cathode material 124 can be in fluid communication with electrolyte solution flowing into inlet 138. Spacers 502 and 504 can be positioned between the gas impermeable material and the H₂ generating chamber and the O₂ generating chamber, respectively. As shown, reactor 102 includes transparent region 506 allows light to be transmitted through the cell to the photocatalysts.

B. Materials

1. Polymeric Materials

As discussed above, the systems of the present invention can be made from transparent or opaque polymeric materials. Non-limiting examples of polymeric materials include thermoset and thermoplastic materials. The polymeric material can include a thermoplastic polymer, such as, for example, polyethylene terephthalate, polycarbonate (PC), polybutylene terephthalate (PBT), poly(1,4-cyclohexylidene cyclohexane-1,4-dicarboxylate) (PCCD), glycol-modified polycyclohexyl terephthalate (PCTG), poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyethyleneimine or polyetherimide (PEI) or a derivative thereof, a thermoplastic elastomer (TPE), a terephthalic acid (TPA) elastomer, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), a polyamide (PA), polystyrene sulfonate (PSS), polyether ether ketone (PEEK), polyether ketone ketone (PEKK), acrylonitrile butyldiene styrene (ABS), polyphenylene sulfide (PPS), a copolymer thereof, or a blend thereof. The polymeric material can comprise a thermoset material, such as, for example, an unsaturated polyester resin, a polyurethane, bakelite, duroplast, urea-formaldehyde, diallyl-phthalate, epoxy resin, an epoxy vinylester, a polyimide, a cyanate ester of a polycyanurate, dicyclopentadiene, a phenolic, a benzoxazine, a co-polymer thereof, or a blend thereof. In a preferred embodiments, the entire or portions of the PEC system is made from PMMA.

Polycarbonate polymers suitable for use in the present disclosure can have any suitable structure. For example, such a polycarbonate polymer can include a linear polycarbonate polymer, a branched polycarbonate polymer, a polyester carbonate polymer, or a combination thereof. Such a polycarbonate polymer can include a polycarbonate-polyorganosiloxane copolymer, a polycarbonate-based urethane resin, a polycarbonate polyurethane resin, or a combination thereof.

Such a polycarbonate polymer can include an aromatic polycarbonate resin. For example, such aromatic polycarbonate resins can include the divalent residue of dihydric phenols bonded through a carbonate linkage and can be represented by the formula:

where Ar is a divalent aromatic group. The divalent aromatic group can be represented by the formula: —Ar₁—Y—Ar₂—, where Ar₁ and Ar₂ each represent a divalent carbocyclic or heterocyclic aromatic group having from 5 to 30 carbon atoms (or a substituent therefor) and Y represents a divalent alkane group having from 1 to 30 carbon atoms. For example, in some embodiments, —Ar₁—Y—Ar₂— is Ar₁—C(CH₃)—Ar₂, where Ar₁ and Ar₂ are the same. As used herein, “carbocyclic” means having, relating to, or characterized by a ring composed of carbon atoms. As used herein, “heterocyclic” means having, relating to, or characterized by a ring of atoms of more than one kind, such as, for example, a ring of atoms including a carbon atom and at least one atom that is not a carbon atom. “Heterocyclic aromatic groups” are aromatic groups having one or more ring nitrogen, oxygen, or sulfur atoms.

In some embodiments, Ar₁ and Ar₂ can each be substituted with at least one substituent that does not affect the polymerization reaction. Such a substituent can include, for example, a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a phenyl group, a phenoxy group, a vinyl group, a cyano group, an ester group, an amide group, or a nitro group.

Aromatic polycarbonate resins suitable for use in the present disclosure can be commercially available, such as, for example, Lexan® HF1110, available from SABIC Innovative Plastics (U.S.A.), or can be synthesized using any method known by those skilled in the art. Polycarbonate polymers for use in the present disclosure can have any suitable molecular weight; for example, an average molecular weight of such a polycarbonate polymer can be from approximately 5,000 to approximately 40,000 grams per mol (g/mol).

2. Electrolyte Solution

The electrolyte solution can be an aqueous solution that has a pH of 0 to 14. In some embodiments, the electrolyte solution is a buffer solution have a pH of 6 to 7.5, or greater than, equal to, or between any two of 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4 and 7.5. The amount of electrolyte solution can be varied to fit the system. In some embodiments, an amount of electrolyte in the H₂ and O₂ reservoirs is minimal. By way of example, the amount of electrolyte is at least 5 vol % of the total volume of the reservoirs. In some embodiments, the amount of electrolyte solution in the reactor is 5 to 100% of the volume of the reactor. The electrolyte solution can be an aqueous solution of inorganic salts. The inorganic salts can have positive (K⁺, Na⁺, NH₄ ⁺, Ca²⁺) and negative (NO₃ ⁻, SO₄ ²⁻, PO₄ ³⁻, H₂PO₄ ⁻, HPO₄ ²⁻) ions that do not involve any kind of redox reaction under water oxidation condition in order to avoid possible redox reaction except pure water splitting reaction. Non-limiting examples of buffer solutions include phosphonium salts, sulfate salts, carbonate salts, and mixtures thereof.

3. Anode, Cathode, and Photocatalysts

Any anode or cathode material known for water-splitting reactions can be used. Non-limiting examples of anode material include metal oxides. Non-limiting examples of cathode material include metals or metal alloys. The metal oxide and metals can include platinum (Pt), cobalt (Co), molybdenum (Mo), nickel (Ni), iron (Fe, tungsten (W), tin (Sn), ruthenium (Ru), irdium (Ir), copper (Cu), manganese (Mn), chromium (Cr), zinc (Zn), cerium (Ce), lanthanum (La) or oxides, or alloys thereof. Non-limiting examples of oxygen evolution catalysts include Ir, Ru, Co, Co/phosphorus (P), CoFe, Cu, Fe, FeMn, Ni, NiCe, NiCo, NiCr, NiFe, NiCe, NiCeCoCe, NiLa, NiMoFe, NiSn, NiZn, or oxides thereof, or combinations thereof. Non-limiting examples of hydrogen evolution catalysts can include, Pt, Co, CoMo, CoNiFe, Fe, FeMo, Mo/sulfur (S), Ni, NiCo, NiFe, NiMo, NiMoC, NiMoFe, NiSn, NiW, or combinations thereof.

The photocatalysts useful in the present invention is suitable to generate H₂ and O₂ from water. By way of example, Z-scheme catalysts using two different semiconductor materials. In a preferred embodiment, the anodic catalyst can include metal oxides and the cathodic catalyst can include metal/metal alloy. Non-limiting examples semiconductor materials include strontium (Sr), titanium (Ti), Co, and thallium (Tl), and arsenic (As). Dopants such as phosphorous (P), sulfur (S) and barium (Ba) can be added. Non-limiting examples of semiconductor-type catalysts include SrTiO₃, BaTiO₃, GaN, CoPS, GaAs, GaAs/InGaP, NiMo/GaAs, InGaP/TiO₂Ni, or combinations thereof. The photocatalysts can have layers of metals, metal oxides, and other materials of various thicknesses (e.g., 1 nm to 300 microns or any value there between. For example, a cathodic photocatalyst can include a bottom Ga layer, a InGaAs layer, a Tl layer, backsurface field layer (BSF), two InGaAs layers, an InGaP layer, a Tl layer, a BSF layer, two InGaP layers, an AlInP layer, and a top layer of InGaAs. In another example, an anodic photocatalyst can include be a p-n junction type catalyst that can include a GaAs layer on a support with InAlP layer, InGaP layer, a InGaP layer, a AlInGaP layer, a AlGaAs layer, an InGaP layer, an InAlP layer, a GaAs layer, a InGa P layer, a GaAs layer, and a Ni substrate layer as the top layer.

Systems 100, 200, 300 and 400 can have photocatalysts arranged as shown in FIGS. 4A, 4B, 7, 8A, and 8B. FIGS. 4A and 4B represent an electrolysis system with the electrodes being attached to a voltage source. In some embodiments, the photocatalyst can be attached to the impermeable material 402 as shown in FIG. 7. This type of catalyst can be a monolithically integrated system and the entire catalyst and any hydrogen and oxygen co-catalysts 702 integrated into a thin film. During use, where one side of the film generates H₂ (cathode) and the other side generates O₂ (anode) as shown in FIG. 7, when exposed to irradiation source 704. Non-limiting examples of these types of photocatalysts are monolithically integrated solar-driven water-splitting devices can be based on tandem, Z-scheme or multi-junction structures.

In some embodiments, photocatalysts can be are used either for generating H₂ or O₂ and they can separated from corresponding counter electrodes. The photocatalysts (e.g., 112 and/or 124) and corresponding electrodes (e.g., 112 and/or 124) can be connected through circuit 126 (e.g. a copper wire). The photocatalyst can be based on tandem, Z-scheme or multi-junction structures. In some embodiments, circuit 126 can be attached to impermeable material 402 or conduit 302. Referring to FIG. 8A, cathodic photocatalyst 802 is attached to support 804 and is positioned in hydrogen generating chamber 106. Cathodic photocatalyst 802 is separated from oxygen generating chamber 108 conduit 302 or impermeable material 402 (not shown). Cathodic photocatalyst 802 is connected to metal (e.g., Pt) anodic electrode 806 via circuit 126 (e.g., copper wire) through conduit 302. When cathodic catalyst 802 is irradiated with a light source H₂ can be generated from H⁺ in hydrogen generating chamber 106, and O₂ can be generated in O₂ generating chamber from voltage applied to the anodic electrode.

Referring to FIG. 8B, anodic photocatalyst 808 is attached to support 804 and is positioned in oxygen generation chamber 108. Anodic photocatalyst 808 is separated from hydrogen generating chamber by conduit 302 or impermeable material 402 (not shown). Anodic photocatalyst 808 is connected to metal (e.g., Pt) cathodic electrode 810 via circuit 126 (e.g., copper wire) through conduit 302. When anodic catalyst 808 is irradiated with a light source, O₂ and electrons can be generated in O₂ generating chamber. The electrons can travel through circuit 126 to cathode 810, which can generate from H⁺ in hydrogen generating chamber 106.

4. Gas Selective Membranes for Gas Phase Separation

Hydrogen selective and oxygen selective membranes used to purify the generated H₂ and/or O₂ can be manufactured or be obtained from commercial sources. Non-limiting examples of commercial membrane sources are Air Products (U.S.A.), Membrane Technology Research, Inc. (U.S.A.), Air Liquid (U.S.A.), UBE Industries, LTD. (JAPAN), or the like.

Non-limiting examples of materials that compose the hydrogen separation membrane include polymeric and carbon membranes. Polymeric membranes typically achieve hydrogen selective molecular separation via control of polymer free volume. Polymeric membranes may be comprised, for example, of glassy polymers, epoxies, polysulfones, polyimides (e.g., polyimide membrane from UBE, or Proteus™ membranes from Membrane Technology and Research, Inc., and other materials, and may include crosslinks and matrix fillers of non-permeable (e.g., dense clay) and permeable (e.g., zeolites) varieties to modify polymer properties. Carbon membranes are generally microporous and substantially graphitic layers of carbon prepared by pyrolysis of polymer membranes or hydrocarbon layers. Carbon membranes may include carbonaceous or inorganic fillers, and are generally applicable at both low and high temperature. The hydrogen separation membrane may be a dense membrane composed only of the above-mentioned materials, or may be a dense thin membrane composed of the above-mentioned materials supported on a porous body. In the case of the former, the thickness of the hydrogen separation membrane is preferably 0.1 μm or more and more preferably 0.5 μm to 5 μm from the viewpoints of mechanical strength and hydrogen permeability. In the case of the latter, the thickness of the thin membrane is 0.1 to 25 μm or more and more preferably 0.1 μm to 2 μm from the viewpoint of processability.

In cases where the hydrogen separation membrane includes the dense thin membrane composed of the above-described materials and the porous body supporting the membrane thereon, the replacement of gaseous species tends to be inhibited on the side of the porous body and, thus, it is preferable for a dense thin membrane to be the side contacted with a mixed gas, and a porous body to be the side contacted with permeated hydrogen.

Oxygen selective membranes can include a perfluorocarbon material, a polysiloxane material, a fluorinated polysiloxane material, a perfluorinated polyethers material, and an alkyl methacrylate-based copolymeric material. Oxygen selective membranes are available from commercial sources. For example, Sepuran® membranes from Evonik Industries (Austria) can be used. In some embodiments, oxygen can be released to the environment.

C. Method of Producing H₂ and O₂ from Water

The water-splitting systems of the present invention can be used to produce H₂ and O₂ from water. With reference to FIGS. 1-6, an electrolyte solution (e.g., electrolyte solution 114) can be provided to H₂ generating chamber 106 and O₂ generating chamber 108 of reactor 102. Purge gas can enter electrolyte solution 140 and the solution can be purged until no or substantially no H₂ and O₂ are present in the electrolyte solution. For example, the solution can include 1 vol. % or less, 0.05 vol. % or less, or 0.005 vol. % or less or undetectable amounts of H₂ and/or O₂. The degassed electrolyte solution 114 can be moved (e.g., pumped or pressurized) from electrolyte source 104 to the H₂ and O₂ generating chambers. The degassed electrolyte solution 114 can include purge gas, but H₂ and O₂ are preferably removed from solution 114 prior to entering the H₂ and O₂ generating chambers. In some embodiments, degassed electrolyte solution 114 is added to the reservoirs independently. By way of example, purge gas can enter H₂ generating chamber through inlet 502 of FIG. 5A. Purge gas can be provided to H₂ generating chamber or the H₂ and O₂ generating chambers continuously or intermittently. By way of example, prior to starting the water-splitting reaction, the electrolyte solution can be added to the chambers and purge gas can be provided to the chambers until the electrolyte solution includes little to no O₂ present as measured using known analytical techniques (e.g., gas detectors). The purge gas can be slowed or discontinued in the H₂ and O₂ generating chambers and/or the electrolyte source for a desired amount of time. Purging can be resumed when detectable amounts of O₂ and/or H₂ are found in the electrolyte solutions exiting the H₂ and O₂ generating chambers, respectively. Such monitoring and addition of purge gas can inhibit and/or prevent H₂ or O₂ cross-contamination in the H₂ and O₂ generating chambers during the water-splitting reaction.

In reactor 102, current and/or electromagnetic radiation can be applied to anode 124 to generated electrons, which travel through circuit 126 to cathode 112 to generate H₂. In some embodiments, both anode and cathode photocatalysts can receive electromagnetic radiation. In other embodiments, voltage and light are applied. When electromagnetic radiation is used, the source of the electromagnetic radiation can be natural (e.g., sunlight) or artificial (e.g., a lamp). A non-limiting example of an artificial source is a UV lamp that provides light at 300 to 400 nm. Excitation of the photocatalyst 112 in the presence of water can generate hydrogen ion (H⁺). Conditions for the water-splitting can include temperature and pressure. The reaction temperature can be greater than, equal to, or between any two of 5° C., 10° C., 20° C., 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C. and 100° C. The reaction pressure can be greater than, equal to, or between any two of 0.01 MPa, 0.1 MPa, 0.5 MPa, 1 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, 2.0 MPa and 2.1 MPa.

As H₂ and O₂ are generated, electrolyte solution 114 having H₂ and O₂ dissolved therein exits chambers 104 and 106 and enters H₂ reservoir 202 and O₂ reservoir 204, respectively. In some embodiments, electrolyte solution 114 exiting H₂ generating chamber 106 passes through a compressor and gaseous H₂ is separated from the electrolyte solution and enters purification unit 212. In some embodiments, a compressor is not used. In H₂ reservoir 202, dissolved H₂ is released from the electrolyte solution producing an electrolyte solution that can have less than 0.2 ppm or 0 to 0.2 ppm dissolved H₂ remaining. Released H₂ can exit H₂ reservoir 202 and enter purification and/or collection unit 212. In some embodiments, released H₂ can be provided directly to other units or used as a fuel. In O₂ reservoir 204, dissolved O₂ is released from the electrolyte solution producing an electrolyte solution that can have 0.2 ppm to 0.4 ppm or greater than, equal to, or between any two of 0.2 ppm, 0.25 ppm, 0.3 ppm, 0.35 ppm and 4 ppm of O₂ remaining. Released O₂ can exit O₂ reservoir 202 and enter purification and/or collection unit 214. In some embodiments, released O₂ can be provided directly to other units for use as an oxidant. Release of H₂ and O₂ can be facilitated by purging, compression, heating or any known techniques to degas an aqueous solution.

The electrolyte solutions can exit reservoirs 202, 204, and enter electrolyte source 104. As shown in the FIGS. 2 and 3B, the solutions are combined prior to entering electrolyte source 104. However, the solutions can each independently enter electrolyte source 104. As the electrolyte solutions are combined the concentration of the H₂ and O₂ can remain under the flammability limit. Combining the electrolyte solutions can restore the pH gradient of the electrolyte solution used in the H₂ and O₂ generating chambers. In electrolyte source 104, the residual or low amounts of H₂ and O₂ are removed from the electrolyte solution and exit electrolyte source 104 via outlet 154. Removal of H₂ and O₂ can be facilitated by purging, compression, heating or any known techniques to degas an aqueous solution. In a preferred embodiment, the remaining H₂ and O₂ are removed by purging with nitrogen gas. The exiting gases can be sent to a collection unit, purification unit or transported to other processing units. The purged electrolyte solution exits electrolyte source (e.g., reservoir) via outlet 138 and enters H₂ generating chamber 106 via inlet 142 and O₂ generating chamber 108 via inlet 144. In some embodiments, the process is conducted in a continuous manner with electrolyte solution being added to the chambers while H₂ and O₂ are being removed. In some embodiments reservoirs 202 and 204 are not used and the H₂ and O₂ are removed from the electrolyte solution in electrolyte source 104.

EXAMPLES

The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1 Generation of H₂ and O₂ from an Aqueous Electrolyte Solution in the Absence of a Purge Gas

A water-splitting system included a reactor having electrolyte (200 mL, 0.1 M Na₂SO₄), an anode (Pt coated-Ni mesh) in an O₂ generating chamber, a cathode (GaAs based triple junction solar cell) in an H₂ generating chamber. The H₂ generating chamber connected to a H₂ reservoir and the O₂ generating chamber were connected to an O₂ reservoir. The reactor and the H₂ and O₂ reservoirs were connected to an electrolyte reservoir. The reactor was irradiated with a solar simulator at the intensity of 1 Sun (100 mW/cm²). The light intensity reaching the reactor was maintained at 100 mW/cm² by adjusting the distance between the lamp and the reactor cell. The distance range was typically between is between 20 cm to 50 cm depending on the desired light flux. The rate of pumping of the electrolyte solution through the system was about 100 mL/min. No nitrogen purge was used. In the absence of a nitrogen purge, 75% H₂/O₂ separation was achieved (Table 1). Solar To Hydrogen (STH) was 7.5% at pH=7 under one sun. Table 1 lists the total water splitting results with multi-junction system using the membrane-less reactor without the N₂ purging during the water-splitting reaction. From the data, it was determined that the dissolved H₂ gas was transferred from the H₂ reservoir (Compartment 1) to the O₂ reservoir (Compartment 2) via the O₂ chamber (See, FIG. 2) and then released to the gas phase until equilibrium was released.

TABLE 1 Compart- Compart- Compartment 1 plus Results ment 1 ment 2 Compartment 2 H₂ (mmol/s) 6 × 10⁻⁶ 2 × 10⁻⁶ 8 × 10⁻⁶ Exposed Area (cm²) 0.25 0.25 0.25 ΔG° (J/mol at 25° C.) 237000 237000 237000 Flux (mW/cm²) 100 100 100 320-1000 nm STH (%) 5.688 1.896 7.584

Example 2 Cross-Contamination of H₂ and O₂ from an Aqueous Electrolyte Solution as a Function of Flow

Using the experimental reactor system of Example, 1 the cross-over of H₂ to the O₂ chamber was studied as a function of nitrogen flow. In this study, H₂ was injected into the H₂ reservoir (C1 in FIGS. 9A & 9B, e.g., H₂ reservoir 202 in FIG. 2) and the amount of H₂ was measured in both reservoirs (C1 and C2 in FIGS. 9A & 9B, e.g., H₂ reservoir 202 and O₂ reservoir 204 in FIG. 2) with gas chromatography as a function of time while circulating water with a flow rate of about 80 mL/min and a N₂ purge rate of about 1 mL/s during the experiment. Injection of H₂ and O₂ simulated the production of H₂ and O₂ from H₂O in larger amounts using the catalytic system of Example 1. At the end of the H₂ injection experiment the whole system was purged with N₂ to remove all the H₂ from the H₂ and O₂ reservoirs. Subsequently, O₂ was injected into the O₂ reservoir (C2) and the O₂ content in the H₂ and O₂ reservoirs was monitored as a function of time. FIGS. 9A and 9B are graphs of addition of H₂ (FIG. 9A) and O₂ (FIG. 9B) into their respective reservoir. In FIG. 9A, the data line 900 the changes of H₂ concentration with time in the H₂-injected reservoir (C1) and data line 902 shows the changes of H₂ concentration in the O₂ reservoir (C2). In FIG. 9B, data line 904 shows the changes of O₂ concentration with time in the O₂-injected reservoir (C2) and data line 906 shows the changes of O₂ concentration in the H₂ reservoir (C1). The results of these experiments showed that after 90 min, around 0.3 vol. % of total H₂ was transferred from the H₂-injected reservoir (C1) to the O₂ reservoir (C2) under the applied experimental conditions as shown in FIG. 9A (i.e., line 902 increases and line 900 decreases). Furthermore, no O₂ crossing was observed (within experimental error) after 180 min as shown in FIG. 9B.

Example 3 Cross-Contamination of H₂ and O₂ from an Aqueous Electrolyte Solution as a Function of Time

Using the experimental design of Example, 3, the performance/efficiency of the separation of H₂ and O₂ of the reactor system in the presence of both gases was evaluated. H₂ and O₂ were injected into H₂ reservoir (C1, FIGS. 10A and 10B) and O₂ reservoir (C2, FIGS. 8A and 8B), respectively, and the amounts of gases were measured in both reservoirs with gas chromatography as a function of time. In FIG. 10A, data line 1000 shows the changes of H₂ concentration with time in the H₂-injected reservoir (C1) and data line 1002 shows the changes of O₂ concentration in the same reservoir (C1). In FIG. 10B, data line 1004 shows the changes of O₂ concentration with time in the O₂-injected reservoir (C2) and data line 1006 shows the changes of H₂ concentration in the same reservoir (C2). From these results, it was determined that after 170 min, a small amount (around 0.5 vol. %) of H₂ gas was transferred from the injected reservoir (C1) to the other one (C2) (See, FIG. 10A), whereas after 140 min about 3 mol. % of O₂ gas was transferred from the C2 to the C1 (See, FIG. 10B).

Based on the data, the low amount of H₂ and O₂ crossing resulted in a H₂/O₂ oxygen mixture with low H₂ and O₂ ratios, which was under the explosion limit (5%). As a result, the H₂ rich gas mixture of H₂, O₂ and N₂ can be further separated by conventional gas separation membrane to obtain high purity H₂ when needed. 

1. A water-splitting system for the production of hydrogen (H₂) gas and oxygen (O₂) gas from an aqueous electrolyte solution, the system comprising: a reactor comprising: a H₂ generating chamber comprising a cathode and at least a first fluid inlet fluidly coupled to a purged electrolyte source, a purge gas source, or a combination thereof; an O₂ generating chamber comprising an anode in electrical communication with the cathode and at least a first fluid inlet fluidly coupled to the purged electrolyte source, the purge gas source, or a combination thereof, and a fluid mover for moving degassed electrolyte solution to the H₂ generating chamber electrolyte inlet and the O₂ generating chamber inlet via degassed electrolyte solution outlet and piping; wherein the piping fluidly couples the H₂ generating chamber with the O₂ generating chamber, and wherein the H₂ and O₂ generating chambers are coupled by one or more apertures.
 2. The water-splitting system of claim 1, wherein the H₂ or O₂ gas permeable material is a membrane.
 3. The water-splitting system of claim 1, further comprising a H₂ reservoir fluidly coupled to the H₂ generating chamber, the purged electrolyte source and an H₂ product outlet.
 4. The water-splitting system of claim 3, further comprising a H₂ purification system, fluidly coupled to the H₂ product outlet.
 5. The water-splitting system claim 1, further comprising an O₂ reservoir fluidly coupled to the O₂ generating chamber, the purged electrolyte source, and an O₂ product outlet.
 6. The water-splitting system of claim 5, further comprising an O₂ purification system, fluidly coupled to the O₂ product outlet.
 7. The water-splitting system of claim 1, wherein purged electrolyte source is fluidly coupled to the purge gas source.
 8. The water-splitting system of claim 1, wherein the first fluid inlets are fluidly coupled to the purged electrolyte source, and wherein the H₂ generating chamber further comprises a second inlet and/or the O₂ generating chamber further comprises a second inlet, each second inlet fluidly coupled to the purge gas source.
 9. (canceled)
 10. The water-splitting system of claim 1, further comprising a conduit coupled to the H₂ generating chamber and the O₂ generating chamber, the conduit comprising a first aperture coupled to the H₂ generating chamber and a second aperture coupled to the O₂ generating chamber.
 11. The water-splitting system of claim 1, wherein the anode and the cathode are comprised in a H₂ and/or O₂ gas impermeable material positioned at least partially between the H₂ generating chamber and the O₂ generating chamber. 12-15. (canceled)
 16. A water-splitting process for the production of hydrogen (H₂) gas and oxygen (O₂) gas, the process comprising: (a) providing an electrolyte solution to each of the H₂ generating chamber and the O₂ generating chamber of the water-splitting system of claims 1 to 8 and 10 to 11, the electrolyte solution comprising water, a purge gas, and an electrolyte; (b) subjecting the electrolyte solution in the H₂ generating chamber and the electrolyte solution in the O₂ generating chamber to conditions sufficient to produce a H₂ containing electrolyte solution in the H₂ generating chamber and an O₂ containing electrolyte solution in the O₂ generating chamber, wherein at least a portion of the generated H₂ is dissolved in the H₂ containing aqueous electrolyte solution, and at least a portion of the generated O₂ is dissolved in the O₂ containing electrolyte solution; and (c) subjecting the H₂ containing electrolyte solution and/or the O₂ containing electrolyte solution to conditions suitable to produce a purge gas containing electrolyte solution, a gaseous H₂ stream, a gaseous O₂ stream, or combinations thereof.
 17. The process of claim 16, further comprising: providing the purge gas containing electrolyte solution to the H₂ generating chamber of the water-splitting system, the O₂ generating chamber of the water-splitting system, or both, wherein the purge gas containing electrolyte solution comprises H₂ and O₂ in a molar H₂/O₂ ratio under the explosion limit; and/or providing a purge gas to the H₂ generating chamber, the O₂ generating chamber, or both; and providing the purged electrolyte solution to the H₂ generating chamber and the O₂ generating chamber.
 18. The process of claim 16, wherein the water-splitting conditions comprise a pressure of 0.010 MPa to 2.1 MPa, a temperature of 5° C. to 100° C., a pH of 0 to 14, or a combination thereof.
 19. The process of claim 16, wherein the purge gas reduces contamination of H₂ into the O₂ containing aqueous electrolyte stream, O₂ into the H₂ containing aqueous electrolyte stream, or both.
 20. The process of claim 16, wherein: step (c) comprises: (i) compressing the H₂ containing aqueous electrolyte solution stream to produce a gaseous H₂ stream and the electrolyte solution comprising the purge gas, or (i) collecting the H₂ containing aqueous electrolyte solution stream in the H₂ reservoir, the O₂ containing aqueous electrolyte solution stream in the O₂ reservoir or both; (ii) separating the H₂ gaseous stream from the H₂ containing aqueous electrolyte solution and the O₂ gaseous stream from the O₂ containing aqueous electrolyte solution, or both; (iii) forming an aqueous electrolyte solution comprising residual H₂, O₂, or both; and (iv) purging the step (iii) aqueous electrolyte solution with the purge gas to form the purge gas containing aqueous electrolyte solution; and/or step (b) further comprises flowing the a portion of the electrolyte solution between the H₂ generating chamber and the O₂ generating chamber through at least one of the apertures of claim
 10. 